1. Field of the Invention
The present invention generally relates to brake apparatus for railway vehicles and, more particularly, to a valve for maintaining brake cylinder pressure for all service brake applications.
2. Description of Related Art
Railroad freight cars have a brake pipe that runs through each car and is coupled therebetween so as to extend continuously the length of the train. The brake pipe is charged with compressed air typically at the head end by a compressor on the locomotive. The compressed air not only provides the pneumatic brake force at the respective cars, but also serves as a communication link via which the car's brakes are controlled from the locomotive by increasing and decreasing the brake pipe pressure. Brake equipment for railroad freight cars utilizes control valves to control the operation of the brake cylinders and brakes for the freight cars, such as the ABDX control valve sold and manufactured by Wabtec Corporation.
When a train brake pipe is fully charged to the pressure setting of the locomotive brake valve device, a natural pressure gradient typically exists in the brake pipe due to leakage caused by wear and other issues resulting in pressure changes with the brake cylinders. Assuming the locomotive brake valve is set to charge the brake pipe to 90 psi, the pressure at each car from front to rear of the train will experience a slightly lower pressure due to leakage and fluid flow resistance as the pressure maintaining brake valve attempts to maintain the leakage. The brake pipe pressure will gradually rise from front to back in seeking the natural pressure gradient consistent with the application of brake pipe pressure at the locomotive. Current control valve technology generally only maintains cylinder leakage below 8 psi via a quick service limiting valve when the cylinder pressure falls below the 8-12 psi limiting valve setting.